In general, forged steel rolls are manufactured, due to their large diameter, by casting large-scaled ingots (steel ingots) by the ingot-making method and forging the ingots. In the large-scaled ingots, a macro segregation called as ghost segregation tends to occur from the center to the vicinity of the surface during casting, and this ghost segregation remains inside the manufactured forged steel rolls as a segregation even after passing through a forging step and a heat-treatment step.
FIG. 1 is a longitudinal sectional view of a general ingot obtained by the ingot-making method. As shown in this figure, V segregation and ghost segregation appear inside the ingot as general macro segregations. The V segregation is formed of V shape in the central part of the ingot, and includes dense V segregation in the upper portion and pale V segregation in the lower portion. Settled crystals exist below the pale V segregation. The ghost segregation, in which C, P, Mn or other alloy components are thickened, is located in an area extending from the outside of the V segregation to a position of about ½ of the radius of the ingot, and has a linear segregation line shape extending in the vertical direction of the ingot.
Since the generation position of the ghost segregation is closer to the ingot surface than that of the V segregation, cracks starting from the ghost segregation can be caused, in the forging and heat-treatment steps following the casting of the ingot, by stresses in processing deformation and thermal stresses in heat treatment to cooling.
Further, forged steel rolls, when the surface of the forged steel rolls is worn or abraded during use, are repaired by cutting the roll surface to restore the smoothness into a regulated range. If the ghost segregation is left in the surface vicinity of the forged steel rolls on that occasion, segregation lines can be exposed to the surface of the rolls by this cutting repair, even if no defects such as cracks are caused in the original manufacturing process. When a roll with exposed segregation lines is used for processing such as rolling, the roll itself becomes unsuited for reuse since the segregation lines are transferred onto a workpiece.
Therefore, it is strongly requested to establish a technique for manufacturing a forged steel roll, which can be stably used over a long period of time without cracking in the forging and heat treatment steps and without exposure of segregation lines by repeated cutting repairs of the surface of the forged steel roll.
When ingots obtained by the ingot-making method are used as a material for forged steel rolls as they are, the quality of the resulting forged steel rolls is noticeably deteriorated, particularly, resulting from the ghost segregation. In this regard, steel ingots obtained by the electroslag remelting (hereinafter referred to as “ESR”) method are generally known to have a solidified structure with less segregation. Therefore, as the material for forged steel rolls, the steel ingots obtained by the ESR method are generally applied.
FIG. 2 is a longitudinal sectional view of a general steel ingot obtained by the ESR method. Inside the steel ingot, freckle defects appear in the vicinity of an area of about ½ of the radius of the steel ingot where the curvature radius of molten steel pool is increased, depending on the depth of the molten steel pool. The freckle defects appearing inside the steel ingots by the ESR method is minor, compared with the V segregation and ghost segregation appearing inside the ingots by the ingot-making method. Therefore, the application of the steel ingots obtained by the ESR method as the material for forged steel rolls holds promise for improving the quality of forged steel rolls in a fashion.
However, the freckle defect is a channel type segregation having the same generation mechanism as the ghost segregation. Thus, even when the steel ingots obtained by the ESR method are used as the material for forged steel rolls, deterioration in the quality of forged steel rolls resulting from the freckle defects becomes obvious, similarly to that resulting from the ghost segregation.
The generation mechanism of freckle defects can be explained as follows.
In a forging process, light elements such as C, P, and Si in steel are micro-segregated between dendrite trees in the course of solidification. Such micro-segregation molten steel is lower in density than bulk (base metal) molten steel since these light elements are thickened, and receives a vertically upward force opposite to the gravity by buoyancy.
Although the micro-segregation molten steel stops between branch-like dendrite trees in the early stage of generation, it is then slightly moved upward by buoyancy, integrated with another micro-segregation molten steel located further upward, and developed into an aggregate of micro-segregation molten steels, whereby its volume is increased. Such micro-segregation molten steel is further increased in volume through further upward movement and promotion of the integration, and ascended by large buoyancy produced thereby while crossing branches of dendrites existing upward and breaking the branches to further collect other micro-segregation molten steels.
This micro-segregation molten steel freezes in accordance with the progress of solidification during ascending between dendrite trees, and remains a segregation line inside the steel ingot, and this emerges as a freckle defect.
It goes without saying that the freckle defect is more likely to occur as the content of light elements in molten steel is larger, from the point of its generation mechanism.
When the dendrite structure that is a solidified structure is coarse, the volume of the micro-segregation molten steel tends to increase, and the freckle defects tend to be coarsened. This is attributed to that, when the dendrite structure is coarse, an upward flow of molten steel is easily generated due to an increased volume of the micro-segregation molten steel which is generated first between dendrite trees and a small resistance when the micro-segregation molten steel starts ascending by buoyancy.
In general, when the radius of a steel ingot is represented by R, freckle defects tend to occur in the vicinity of R/2 of the steel ingot where the curvature radius of molten steel pool is increased to facilitate apical extension of dendrite arm spacing. However, when the steel ingot is large-sized and high in the content of light elements, the freckle defects tend to be generated also near the surface of the steel ingot, causing a problem such as generation of cracks in the heat treatment step, similarly to the case of the above-mentioned ghost segregation.
As described above, it is strongly requested to establish the technique capable of preventing generation of cracks in the forging and heat treatment steps, in manufacturing of forged steel rolls, and preventing segregation lines from being exposed even when the surface of the forged steel rolls is repeatedly repaired by cutting, so that the forged steel rolls can be stably used over a long period of time. To meet this request, it is necessary to perfectly suppress the freckle defects in the casting stage of steel ingots or sealing the freckle defects at least nearer the center in relation to the surface of the steel ingots.
It is supposed that the generation of freckle defects can be suppressed by miniaturizing the dendrite structure, from a standpoint of its generation mechanism. Although the miniaturization of the dendrite structure can be attained by increasing the cooling rate in casting, even the manufacturing of small-diameter steel ingots at high cooling rate, for example, involves problems such as restrictions on roll diameter of product and an insufficient forge ratio in forging of the steel ingots.
Patent Document 1 describes a method for miniaturizing the dendrite structure by setting the content of P to 0.025 to 0.060 wt %, as a method for improving the surface roughing of a work roll for cold rolling mill since the surface roughing of the roll is caused by the dendrite structure generated during casting. However, since P is generally an impurity element, and causes embrittlement of iron and steel material, it is not preferred to increase the content of P. Further, P is a light element which causes freckle defects as described above, and an increased content of P is considered to encourage the generation of freckle defects.
Patent Document 2 proposes a determination method in a simulator for casting process, which is characterized by simultaneously evaluating a freckle defect evaluation index (Ra number (Rayleigh number)) with consideration for a segregation molten steel flow, or a hetero-crystal defect evaluation index with consideration for a hetero-crystallization mechanism from the concentration or temperature calculated in a casting process simulation based on an optional casting plan to determine the quality of the casting plan. As described in [0057] of this document, although it can be suggested from the calculation example of FIG. 12 in this document that freckle defects are likely to occur at a site where Ra number is 0.07 or larger, defect evaluation reference values must be newly set when the casting material is changed.